There is an ongoing need for component miniaturization in radio communication devices. For example, smaller and more efficient components are needed for radio communication infrastructure (base stations) including cellular telephones, pager devices and other devices for promoting rapid, efficient and flexible voice and data communication. Delay lines are needed for a variety of such communications applications wherein small size, low cost and high performance are simultaneously required.
Surface acoustic wave (SAW) in-line delay lines are a popular choice for radios because of their low loss and small size advantages. These in-line SAW delay lines are fabricated with in-line interdigital transducers on piezoelectric substrates wherein the propagation of a surface acoustic wave from an input transducer to an output transducer gives rise to delays ranging from about 500 ns to over 10 .mu.s. Unfortunately, where shorter delays are required such as in linear power amplifiers that employ feedforward techniques to reduce distortion, the input and output transducers are in close proximity to each other which results in a direct electromagnetic feedthrough spurious signal, which is difficult to suppress. In addition, an in-line SAW configuration generates "triple transit" and other spurious reflection signals. All of these spurious signals cause amplitude and phase ripples within a passband of the SAW device, and it is difficult to achieve a linear phase output to less than about 2.5 degrees.
Alternatively, a coaxial transmission line can be used to provide an electromagnetic delay line. Coaxial transmission line provides good phase linearity. However, since the speed of a signal in coaxial transmission line is about 100,000 times that of an acoustic signal in a SAW device, a much longer signal path is needed to provide the requisite delay. For example, to provide a 25 ns delay a coaxial cable length of about 20 feet is needed. Such a cable is very expensive and bulky, even when coiled upon itself. Of course, longer delays require longer cable and higher cost. This is a major disadvantage towards the development of miniature base stations, but has been the only option where delay times less than 100 ns are needed.
What is needed is a delay line configuration that provides the phase linearity needed in today's radio communication devices while having a short delay time. In addition, low signal loss and suppressed spurious signal generation would be an advantage. It is also desirable to provide a delay line that can be implemented at a low cost and in a compact form.